ci(.github): add a workflow to backport PRs manually

[jijiechen]

Motivation

To make backport PRs to other branches more convenient, now manually.

Implementation information

changed the way to get PR info from the GitHub Actions context to by invoking the gh CLI

[github-actions]

Reviewer Checklist

🔍 Each of these sections need to be checked by the reviewer of the PR 🔍:
If something doesn't apply please check the box and add a justification if the reason is non obvious.

• Is the PR title satisfactory? Is this part of a larger feature and should be grouped using > Changelog?
• PR description is clear and complete. It Links to relevant issue as well as docs and UI issues
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as an image registry)
• IPv6 is taken into account (.e.g: no string concatenation of host port)
• Tests (Unit test, E2E tests, manual test on universal and k8s)
  • Don't forget ci/ labels to run additional/fewer tests
• Does this contain a change that needs to be notified to users? In this case, UPGRADE.md should be updated.
• Does it need to be backported according to the backporting policy? (this GH action will add "backport" label based on these file globs, if you want to prevent it from adding the "backport" label use no-backport-autolabel label)

[jijiechen]

test run:

• https://github.com/jijiechen/kuma/actions/runs/13558371080
• https://github.com/jijiechen/kuma/actions/runs/13515684486

A example PR created by a run mentioned above: jijiechen#20

[lahabana]

Could be good to do this as well while we touch this job: https://github.com/Kong/team-mesh/issues/295

[jijiechen]

In the latest update, it now extracts change log from original PR.